Description of the Prior Art
In a system of synchronous communication between data processing equipments one of the most widely used methods for sending messages consists in defining a frame structure characterized by:
a synchronization flag,
special coding of the data to be transmitted so that the synchronization flag cannot be recognized in the middle of the data stream transmitted.
One of the best known ways to implement this method is to choose a flag which is a constant stream of P binary zeros followed by a binary 1. The data is then coded simply by inserting a binary 1 each time that a series of (P-1) binary 0 has been transmitted. For example, if the flag is `00001`, the message `0010 0000 10` is transmitted in the form: `00001 0010 00100 10`. The six underlined digits represent the synchronization flag and the inserted binary 1 (the spaces are included only to facilitate reading).
This method has a drawback: the time to transmit a message depends on its contents, which is a serious problem if a fixed routing time is required.
The known solution to this problem is to insert a binary 1 every (P-1) data bits transmitted: it is then certain that P consecutive binary zeros will never be encountered and the transmission time is always the same, regardless of the data transmitted. A well-known example of this method is the use of V.110 frames as defined by the CCITT (Comite Consultatif International du Telephone et du Telegraphe). These frames comprise a flag made up of eight binary zeros followed by a binary 1, a binary 1 being then inserted every seven bits to form a frame of 80 bits, 17 bits used for synchronization and 63 bits for the data.
A frame can be represented by a table with P columns and L rows. The first row, known as the locking row, includes P binary zeros and subsequent rows, known as data rows, each comprise a synchronization bit of binary 1 followed by (P-1) data bits.
When data is transmitted by means of such frames the transmission channel often has a data signaling rate which is greater than that required to convey the frames produced by an equipment.
To economize on transmission channels it is natural to use multiplexing whereby the channels are divided into subchannels so that the data signaling rate of one subchannel conveys the data from one equipment.
Patent application EP-A-0 222 544 proposes a fixed format frame divided into a plurality of subchannels at the same data signaling rate, a specific subchannel being used for synchronization. This solution is restricted to the transmission of subchannels at a specific data signaling rate. It can be necessary to use the same frame to transmit different data signaling rates. The method described in this document can be simplified to define a frame as the succession of a synchronization channel and two subchannels called half-rate subchannels. Also, it is assumed that the combination of the two half-rate subchannels constitutes a full-rate channel.
Provision can therefore be made to use this frame either to transmit data from one equipment requiring a full-rate channel or to transmit data from two separate equipments each requiring a half-rate subchannel.
However, on receiving a frame of this kind it is impossible to tell which of these two situations applies.
Furthermore, this solution cannot be applied if the two half-rate subchannels are not identically synchronized, as there is only one synchronization subchannel.
U.S. Pat. No. 5,113,391 describes a frame capable of conveying channels at different data signaling rates and means for determining the nature of these channels. Here again, however, the system is a synchronized system and all the channels are referred to the same clock.
Thus a serious problem arises in transmitting channels whose time synchronization is unknown.
One obvious solution to this problem is to use a V.110 type frame for a full-rate channel and to use a similar but shortened frame for each half-rate subchannel, two subchannels being transmitted by multiplexing two shortened frames.
In a conventional solution this multiplexing is carried out in such a way that the bits of the channel correspond to the alternating sequence of bits from each of the two subchannels. Thus the odd-numbered bits of the channel come from a first subchannel and the even-numbered bits from a second subchannel.
One channel can be used in this way to convey two subchannels, but this rules out the interchangeable use of two half-rate subchannels or a full-rate channel, because the simultaneous reception of two subchannels can be interpreted as the reception of a single full-rate channel.
This ambiguity results from the fact that if the two subchannels simultaneously transmit a number of binary 0 at least equal to P/2 (where P is the length of the locking row for a full-rate channel), the stream of bits resulting from multiplexing of the two subchannels includes at least P consecutive binary 0 and can therefore be wrongly interpreted as a locking row of a full-rate channel.
Two situations can arise.
In the first situation the locking row used for the subchannels comprises P' binary 0, P' being such that 2.multidot.(P'-1) is greater than or equal to P. Ambiguity arises whenever two rows of data which comprise a series of (P'-1) binary 0 are sent with a time difference less than (P'-INT(P/2+1)+1), INT(x) denoting the integer part of x. After multiplexing of these two rows of data there will be at least P consecutive binary 0.
With P=16 and P'=11, for example, there will be at least 16 consecutive binary 0 in all configurations for which the time difference is less than or equal to the maximal time difference. The two configurations corresponding to this maximum time difference, the initial configuration and the final configuration, are shown in FIGS. 1A and 1B, in which:
the first line represents a first half-rate subchannel,
the second line represents a second half-rate subchannel,
the pattern inside the bold line frame represents the full-rate locking row resulting from regarding the two half-rate subchannels as a single full-rate channel.
In a second situation, ambiguity can arise in circumstances additional to those described above. If the subchannel locking row comprises P' binary 0 where P' is such that 2.multidot.(2.multidot.P'-1) is greater than or equal to P, the end of a frame can include (P'-1) consecutive binary 0. The next frame beginning with a locking row will include P' binary 0 and there will be (2.multidot.P'-1) consecutive binary 0 which will be wrongly interpreted as the locking row of a full-rate frame if the same phenomenon occurs at the same time on both the half-rate subchannels.
FIG. 2 shows on example of a configuration of this type with P=16 and P'=5. The drawing conventions are the same as for FIG. 1 and the separation of two consecutive frames is shown by a vertical line.
It can therefore be seen that the choice of the number P' of binary 0 in the locking row of frames sent on the half-rate subchannels greatly influences the risk of confusion with a locking row of the frame sent on the full-rate channels. The ambiguity can be avoided by reducing sufficiently the number P'.
However, reducing the length of the locking row is achieved to the detriment of transmission efficiency.
The raw data signaling rate is usually defined as the number of bits of the frame transmitted per unit time and is therefore proportional to P'.multidot.L', where L' represents the number of rows of the half-rate frame. The usable data signaling rate is defined as the number of data bits of the frame transmitted during the same unit time and is therefore proportional to (P'-1).multidot.(L'-1). The transmission efficiency is the ratio of the usable data signaling rate to the raw data signaling rate, that is: EQU (P'-1).multidot.(L'-1)/P'.multidot.L'
The closer together the values of P' and L', the greater the efficiency. Thus if a low value is chosen for P', a low value should also be chosen for L'. On the other hand, the longer the frame, i.e. the greater the product of the number of columns by the number of rows (P'.multidot.L'), the higher the transmission efficiency. It is therefore not desirable to reduce the length of the locking row too much.
Consideration could be given to coding a specific field of the half-rate frame so that after multiplexing it is impossible to encounter the value of this field in a full-rate channel.
Although in practise it may be a relatively simple matter to solve this problem if there are very few ambiguous configurations (one only, for example), the problem becomes virtually insoluble with a greater number of cases of ambiguity. It can even be more beneficial to reduce the value of P' because of the number of bits to be added to the half-rate frames to resolve the ambiguity.
An object of the present invention is thus to reduce the number of ambiguous configurations of transmission frames regardless of the length of the locking row, or even to eliminate such ambiguous configurations entirely.